[CAS] Do not create redirect file system when using clang-include-tree

Redirecting file system can canonicalize the file path before forwarding
the path to IncludeTreeFileSystem, which is a simplied FS that can only
intepret the paths that has been seen by dep-scanner. Since all files
that need redirecting already added to underlying FS via DepScan, there
is no need for such layer when compiling using clang-include-tree.

rdar://119727344

lib/Frontend/CompilerInvocation.cpp

@@ -1687,6 +1687,7 @@ static bool ParseClangImporterArgs(ClangImporterOptions &Opts, ArgList &Args,
     // Only set UseClangIncludeTree when caching is enabled since it is not
     // useful in non-caching context.
     Opts.UseClangIncludeTree = !Args.hasArg(OPT_no_clang_include_tree);
+    Opts.HasClangIncludeTreeRoot = Args.hasArg(OPT_clang_include_tree_root);
   }
 
   return false;